The present invention relates to an acid plating bath for electrodepositing bright zinc on a substrate in which the zinc deposit produced thereby is ductile and presents a glossy to bright coating on the substrate over a wide cathodic current density range. The invention relates also to a method of electrodepositing bright coatings on a substrate. More particularly this invention relates to improved zinc plating bath compositions which are free or substantially free of ammonium ions, and the method of plating with such baths.
Alkaline cyanide-containing zinc baths have been most widely used baths for electrodepositing bright zinc plate on substrates. These baths have various disadvantages due particularly to the cyanide content which is constant danger for the people working with the baths. Moreover, the baths are subject to aging due to the decomposition of the cyanide, and the waste water must be subjected to a costly treatment to destroy the cyanide ions and to precipitate zinc as hydroxide prior to disposal.
Alkaline solutions containing complex compounds of zinc and alkaline metal pyrophosphates have been proposed as replacements for cyanide baths and cyanide processes for the electrodeposition of bright zinc. The electrodeposition of zinc using a pyrophosphate bath, however, may result in relatively poor low current density coverage, spore formation, roughness and insufficient brightness. The use of phosphates also may produce waste disposal problems since phosphates are not easily removed and may promote the growth of undesirable aquatic plant life if discharged into streams. These disposal disadvantages limit the acceptance of the pyrophosphate zinc plating bath compositions in industrial applications.
The enactment and enforcement of various environmental protection laws, particularly with respect to improving water quality, have made it desirable and necessary to reduce or eliminate discharge of cyanides, phosphates and a number of metal ions contained in the waste water from plating plants. Accordingly, non-polluting bright zinc plating processes have been sought as alternatives to the zinc cyanide and phosphate-containing baths.
Acidic plating baths have been known, and these baths are cyanide-free. The acid zinc plating baths are much less toxic than the alkaline zinc plating baths and are preferred where the disposal of waste presents an environmental problem. However, many of the known acid zinc plating baths do not produce bright zinc deposits unless the baths contain complexing agents for zinc. Examples of complexing agents which have been utilized in acid zinc baths include organic zinc complexing agents such as hydroxycarboxylic acids or salts thereof, ethylenediamine tetraacetic acid or salts thereof or similar materials which prevent the precipitation of zinc from the bath as the insoluble hydroxide at higher values of pH. Almost all of the baths of the prior art used for depositing bright zinc coatings within the mild acid range also contain large quantities of ammonium salts such as ammonium chloride. The complex forming properties of these ammonium salts and organic salts which is desirable in the plating bath itself is, however, a problem in disposal of the used acid bath. Such complexing agents impede the removal of zinc ions from the waste electrolyte by precipitation in the weakly alkaline pH range and may require special measures for effecting the precipitation of the metals from the bath prior to disposal. Also, the presence of ammonia in the plating bath is undesirable because of the biochemical oxygen demand caused by ammonia, and the chlorine demand of the ammonia in waste treatment facilities.
Acid zinc plating baths which are substantially free of ammonium ions have been described in U.S. Pat. No. 3,928,149. These baths contain, in addition to zinc ions, and an aromatic carbonyl compound, a polypropoxyether having a molecular weight of from about 300 to 1500 and/or a polypropoxy-ethoxy ether having a molecular weight of about 1,000 to about 5,000 and containing up to a maximum of 25% by weight of oxyethylene units. Such baths are reported to produce bright zinc deposits in the absence of ammonium salts.
U.S. Pat. No. 3,920,528 also describes electrolytes free from significant amounts of complexing agents that would impede precipitation of zinc ions at weakly alkaline pH. The plating bath described in this patent contains zinc ions, an inert salt for improving the conductivity of the electrolyte, and a brightener. The preferred brighteners are derivatives of nitrogen-containing heterocyclic rings such as pyridine-3-acetic acid and pyridine-3-sulfonic acid.